This invention relates to a process for heat sealing flexible polyurethane foams prepared from 4,4,4-trichloro-1,2-epoxybutane and/or epihalohydrin-based polyols.
In many applications in which flexible polyurethane foams are used, it is desired to emboss the foam so that it adheres to itself along a design or to adhere the foam to another substrate.
Several heat sealing methods are known in the art which do not require an adhesive. One such method, which is particularly desirable in view of its speed and ease of application, is the so-called dielectric heat sealing method. By applying radio frequency to a material containing polar substituents, the dipole rotates causing friction heating. By applying the electric field and compressing along a design, the foam will melt along the design. The melting foam may also form a bond with a substrate, such as a seat or cushion cover.
There are three parameters which are relevant in a suitable product:
1. foam processing
2. foam properties (compression set)
3. dielectric heat sealability.
A flexible foam recipe requires the use of a blowing agent. If the blowing agent is trapped in the spaces of the foam, the foam will shrink as it cools. It is, therefore, necessary to open a number of the cells to allow the blowing agent, CO.sub.2 for example, to escape. The ability to open the cells is dependent on the degree of cross linking. A highly functional polyol causes extensive cross linking which makes it difficult to open the cells to avoid shrinkage.
The chief foam property of concern in the instant invention is compression set. Compression set is the percentage of the foam which does not recover after being compressed. A 90% compression set means only 10% of the foam recovered. A low compression set is desired. An embossed foam loses its aesthetic value if the portion of the foam not sealed does not stand out. If a cushion sealed to a cover has a high compression set, the cover will have no support and thus result in an excess of cover material.
The sealability characteristic in dielectric heat sealing is attained by the rotation of the polar groups which generate heat by friction. Steric hindrance deters the ability of the polar groups to rotate. Extensive cross linking may also deter polar rotation. If an insufficient number of polar groups are present, the heat seal or melt strength will be irregular or ineffective. On the other hand, an excess of polar groups in foams generally results in a high compression set.
One of the early attempts at producing a dielectrically heat sealable foam incorporated a polyvinyl chloride dispersion in the foam. The solids in dispersions, however, tend to migrate or settle. The polar groups tended to disperse irregularly and unpredictably.
U.S. Pat. No. 4,060,439 attempted to improve the dispersibility of the polar groups in the foam by the use of a polymer polyol. This resulted in a better dispersion of the solids, however, some settling of the solids did occur. This leads to foam wherein a sample from a top layer will require a higher frequency and temperature to attain a seal than a lower layer.
Another attempt at producing dielectrically heat sealable foams, U.S. Pat. No. 3,674,718, uses a cyanoethylated polyol. The polyol contains 5 to 30%, by weight, of cyanoethyl groups. When 30% of the cyanoethyl groups are used there are sufficient polar groups to attain a good seal; but, unlike the dispersions of the prior art the high percentage of cyanoethyl groups have a tendency to degrade upon heating before the melt temperature is reached. The cells of the resulting foams cannot be opened sufficiently and shrinkage occurs. If only 5% cyanoethyl groups are used, a good foam is obtained but there are insufficient polar groups to get a good seal.
The block polyols used in the instant invention result in foams with reproducible dielectric heat sealability results throughout the foam, unlike the dispersions used in the prior art. Also the degree of cross linking is controlled by the low functional block polyols.